This invention relates generally to nuclear magnetic resonance imaging (MRI), and more particularly the invention relates to motion analysis and imaging of an organ such as the heart using phase contrast MRI maps of tissue velocity in the organ.
Methods to calculate the cyclical motion of a portion of an object using phase contrast cine MRI are known. Basically, the phase contrast MRI technique provides maps of vector velocity in the object throughout the motion cycle.
A basic concept is described in U.S. Pat. No. 5,195,525. With this technique the operator selects a region to be tracked by identifying its location in the first frame of the cine image set. The velocity in the region in the first frame is used to calculate its expected location in the second frame. The vector velocity in the second frame at the new location as portrayed in the cine set is used to calculate the position in the third frame, and so on. This tracking can be performed in 3D and promises to yield important information about cardiac motion noninvasively. Higher order integration methods can be used to improve the motion estimate.
An improvement to this method integrates the velocities in both the forward and backward temporal directions. This method, called forward-backward integration, has improved precision and accuracy and is described in "Tracking of Cyclical Motion with Phase-Contrast Cine MRI Velocity Data," JMRI, Vol. 5, pp. 339-345, 1995, and also in U.S. Pat. No. 5,257,626. Another improvement, called forced closure, estimates the additive velocity errors that might be present and corrects for these, improving the quality of the computed motion. This concept is described in U.S. Pat. No. 5,257,625.
Another method for integrating the motion from the velocity data is described in a pending U.S. Pat. application Ser. No. 08/381,801, "Iterative Method of Determining Trajectory of a Moving Region in a Moving Material Using Velocity Measurements in a Fixed Frame of Reference," filed Feb. 1, 1995 by Samuel M. Song and Norbert J. Pelc. In this method, a first trajectory estimate, for example as produced by the forward-backward method, is iteratively refined to arrive at a trajectory that is most consistent with the velocity along the trajectory. In contrast to the other techniques which are recursive, the Song method arrives at the trajectory estimate for one iteration using only the trajectory at the previous iteration and the measured velocity data.
These methods have variable tendency to underestimate motion due to the limitations of the integration model order used. There is a more serious tendency to underestimate motion with these methods due to the limited temporal resolution of the acquired cine-PC data, specifically due to the linear interpolation used in the cine processing. The present invention overcomes these limitations and provides more accurate motion measurements.